The present invention relates to permanent magnetic materials and the method of making the permanent magnet. More particularly, the invention relates to high performance sintered intermetallic materials containing an iron-boron-rare earth composition enriched with praseodymium.
Use of high performance permanent magnets of the iron-boron-rare earth type (Fe-B-RE), where RE is a rare earth element containing concentrations of neodymium (Nd) greater than 95%, has become common in the computer and medical industry since the 1980's. For example, computer hardware manufacturers who manufacture small footprint, large capacity computer data storage and retrieval hardware, use very high performance iron-boron-neodymium permanent magnets (the total rare earth being greater than 99 percent Nd). Additionally, medical devices, such as magnetic resonance imaging (MRI) devices, employ vast quantities of permanent magnetic iron-boron-neodymium material, which contain greater than 90 percent of the total rare earth being neodymium. The prevailing practice of manufacturers of high performance permanent magnets of the iron-boron-neodymium type is to utilize 99.9% or higher concentration of pure neodymium as the rare earth component. These magnets achieve intrinsic coercive forces (Hci) in excess of 8 kilo Oested (kOe) and maximum energy products (BH).sub.max in excess of 30 Mega Gauss Oested (MGOe).
Accordingly, due to the sale of these devices with permanent magnets using 90 percent or greater neodymium as the rare earth component, the worldwide demand for neodymium has increased. As a result, the cost of the raw material neodymium has greatly increased. A real need has arisen to develop iron-boron-rare earth magnets of substantially equal performance, which utilize less neodymium to reduce the cost of manufacture of the permanent magnets and the devices which contain the permanent magnets.
Permanent magnets of the Fe-B-RE type, where RE is one or more rare earth elements of which at least 50% of RE is neodymium and/or praseodymium (Pr), are known. U.S. Pat. Nos. 4,684,406 and 4,597,938 teach a high performance magnet consisting of, by atomic percent, (i) 12.5 percent to 20 percent RE wherein RE is at least one rare earth element selected from the group consisting of neodymium, praseodymium, lanthanum, cerium, terbium, dysprosium, holmium, erbium, europium, samarium, gadolinium, promethium, thulium, ytterbium, lutetium and yttrium and at least 50% of RE consists of neodymium and/or praseodymium; (ii) 4 percent to 20 percent boron; and (iii) the balance iron with impurities. Additionally, as may be seen from these patents and U.S. Pat. No. 4,975,130, a method of making the magnet is taught by forming powders of the alloys of the above composition; melting the powders to form an ingot; pulverizing the ingot to form an alloy powder having a mean particle size from 0.3 to 80 microns; compacting the powder at a pressure of 0.5 to 8 Tons/cm.sup.2 ; subjecting the compacted body to a magnetic field of about 7 to 13 kOe; and then sintering at a temperature between 900 to 1,200.degree. C. A permanent magnet prepared in the above fashion specifically comprised of by atomic percent 77Fe-9B-9Nd-5Pr, sintered at 1,120.degree. C. for four hours in an inert atmosphere can acquire a maximum energy product (BH).sub.max of 31.0 MGOe. Likewise, a permanent magnet comprised of by atomic percent 79Fe-7B-14Nd, sintered at 1,120.degree. C. for one hour, can acquire a maximum energy product (BH).sub.max of 33.8 MGOe.
U.S. Pat. No. 4,908,078 shows a rare earth magnet article consisting of the three rare earth elements neodymium-praseodymium-cerium within defined atom ratios of each in the formula: (Nd.sub.1-(p+q) Pr.sub.p Ce.sub.q).sub.x B.sub.y Fe.sub.1-(x+y) wherein 0.1.ltoreq.x.ltoreq.0.3, 0.02.ltoreq.y.ltoreq.0.09, 0.1.ltoreq.p.ltoreq.0.3, and 0.02.ltoreq.q.ltoreq.0.15. Praseodymium is 10 to 30 percent of the total rare earth and cerium is 2 to 15 percent of the rare earth with the balance neodymium. The resultant magnet has a coercive force (Hc) of at least about 5 kOe and a residual magnetic flux density (Br) of at least about 10 kiloGauss (kG). Also, U.S. Pat. No. 5,129,963 discusses rare earth magnet alloys with excellent hot workability having compositions in atomic percent of 10 to 16 percent rare earth elements, 3 to 10 percent boron and about 74 to 87 percent iron (with or without cobalt) where the rare earth elements are neodymium and/or praseodymium plus up to 20 percent of the rare earth is selected from cerium, lanthanum and/or yttrium.
The aforementioned prior art patents fail to disclose or suggest what significance the amount of praseodymium greater than 50 percent of the total rare earth in the presence of cerium, lanthanum, and/or yttrium may have on the magnetic performance of a iron-boron-rare earth magnet. Nor does the prior art teach or suggest in substitution for neodymium, ranges of concentrations of cerium which may form part of the rare earth component with greater than 50% praseodymium that will give equal or better magnetic performance than the iron-boron-neodymium magnets described above. Thus, due to cost and performance considerations, there is a need for a praseodymium-rich permanent magnet of the iron-boron-rare earth type which may further contain other rare earth elements that performs equally or better than the known Fe-B-Nd permanent magnets and which is useful in devices such as magnetic resonance imaging instruments.